-The Chemical Plant zone boss everyone else mentioned
-Gliding over the studiopolis act 1 boss trigger
-In one of the special stages, I think the 2nd or 3rd one, you can either go up a ramp over a gap in the path, or jump over the ramp and onto a ring block which then bounces you up over the gap as well. But one time hit the block, but I still fell through. I was like "Damn, guess you've gotta hit it just right", and the camera panned down as if I fell out of the course, but I could still hear jumping sounds when I pressed the button and then the "course out" message or whatever it is flashed on screen as if I hadn't yet fallen out. So I'm wondering whether I did actually make it but the game thought I hadn't so fixed the camera downward below the course when I was actually still playing and then accidentally jumped out of the course since I couldn't see.

Well, if you know you'll just one-shot it and forget it, then yeah, I'd say wait. If you know you'll be trying for all the achievements or know you'll be replaying stages just because you like to, then now is as good a time as any.

I've been hunting down those S-Ranks, and have come to realise there's a lot more in terms of collectables than I thought in this game.
All Red Rings unlock the five numbered rings in a stage. Complete those, and the moon medals become available. And that's on all stages. And that doesn't count the missions that unlock all the avatar equipment - those will seriously take some time.
But with all that collecting, it still gives a chance to enjoy the levels themselves, and that really is the whole point of a Sonic game.

That pretty much sums up the theme of the game. If the story wasn't based on the end of the world, Classic was removed from it, and the entire idea of the resistance, that theme would still ring true. This game is thematically saying don't turn your back on help just because you don't know the one offering it.